The present invention relates to a brake system, and more particularly, to a method and apparatus for sensing travel of parking brake release system.
Most vehicle designs incorporate parking brakes. Typical parking brake configurations employ regular drum brakes on a rear wheel with a simple mechanical linkage to engage brake shoes to their respective drums. For instance, an actuator may pull a steel brake cable taut in response to an operator depressing a pedal, lever or button. The resultant tension on the cable is transferred to the end of a brake lever. Other cables may draw the brake shoes firmly against the drums in response to rotation of the lever. A release knob or button causes the actuator to return to its initial position, reintroducing slack into the cables and, consequently, disengaging the brake shoes.
The reliability and integrity of brake performance depends largely upon the manner in which the actuator releases tension in the brake cable. For instance, should the actuator release too much cable over-release will result. This condition requires the actuator to reel in excessive cable slack prior to a next application, translating into system delays and damage. Conversely, too little release of the cable may result in a brake drag. Such a condition occurs when the brake pads do not fully release. Brake drag retards vehicle performance while causing overheating and damage to brake pads and other system components.
To reduce the occurrence of over-release and drag, some brake manufacturers incorporate systems that use either position or periodic force adjustments to regulate cable release. In particular, one type of position adjustment is made by a position adjuster proximate to the actuator that periodically retracts cable slack to compensate for lining wear and stretching. Another type of position adjustment is made by a position-based release mechanism that consistently moves the actuator a preset distance that ideally corresponds to an optimum brake release point. The preset distance needs to be manually adjusted at service intervals to compensate for system wear factors. In practice, even if serviced regularly, cable stretching and other aging factors cause release points to substantially migrate in between adjustments, allowing the problems of over-release and brake drag. In addition, some users do not service the braking system regularly.
Closed loop position feedback is known in other types of powered mechanisms. In particular, improved position adjustment is achieved with linear position feedback from a sensor (e.g., linear variable displacement transducer (LVDT), linear resistive potentiometers, and optical sensors). However, these sensors are expensive and prone to reliability limitations due to wear and contaminants.
Consequently, a significant need exists for an improved powered parking brake system that avoids reliability degradation due to over-release of brake drag.
The present invention addresses these and other problems associated with the prior art with linear position sensing of a powered parking brake mechanism that increases reliability and thus reduces costs of servicing and repair. In particular, non-contact sensing of linear position of an actuator that linearly positions the parking brake cable readily identifies coarse linear position without extensive computational circuitry and processing.
Consistent with one aspect of the present invention, a position sensor, and a parking brake release mechanism that uses the position sensor, determine linear position by utilizing a plurality of Hall effect transducers that are arrayed to detect a magnet coupled to the parking brake release mechanism. The reliable performance of the Hall effect transducers increases reliable sensing of linear position.
Consistent with another aspect of the present invention, longitudinal spacing of the Hall effect transducers in combination with a known movement of the parking brake release mechanism advantageously indicates coarse linear position by detecting not more than one transducer that is operating in its positively sloped region. A simple comparison of output signals between two positions of the cable thus readily identifies the coarse linear position.
The above and other objects and advantages of the present invention shall be made apparent from the accompanying drawings and the description thereof.